.container{
    position: absolute;
    top: 50%;
    left: 50%;
    margin-left: -200px;
    margin-top: -100px;
    width: 400px;
    height: 200px;
    background-color: #ccc;
}
.left-top{
    background-color: #fc0;
    border-bottom-right-radius: 50px;
    width: 50px;
    height: 50px;
}
.right-bottom{
    position: absolute;
    right: 0;
    bottom: 0;
    background-color: #fc0;
    border-top-left-radius: 50px;
    width: 50px;
    height: 50px;
}